1. Field of Endeavor
The disclosure relates generally to turbines and specifically to rotors and rotor blades that are rotationally fitted therein.
2. Brief Description of the Related Art
Known fastening arrangements for fitting blades into rotors to form a blade row include pinned roots and side entry fir trees. Each of these configurations requires side access, which, in steam turbines, places limitations on the steam path design. An alternative structure for fitting blades that does not have this disadvantage uses a so-called straddle root. While this does not require side access, a fitting window in the rotor is required and this window creates a weak point. A yet further blade fitting involves rotational fitting.
Rotationally fitted blades may have either T- or L-shaped roots as, for example, disclosed in U.S. Pat. No. 5,236,308. Both the T- and L-shaped roots may be rotationally fitted and fixed into a complimentary shaped channel. As the axial length of the root is typically greater than its circumferential width, the space required to rotational fit a root is greater than the circumferential space it requires when it is operationally aligned. In order to create additional fitting space, the blade's roots may be configured for over-rotation in the channel, as, for example, described in GB 2 171 150 A by having a parallelogram shaped platform and/or root and further by reducing the circumferential width of the root, below its required width, and then filling the resulting gap, after fitting of the all the blades of a blade row, with shims. Alternatively, as described in U.S. Pat. No. 3,567,337, the blade root foot and rotor slot may be configured to each include at least one lateral surface which is sloped so as to engage the blades in opposition to centrifugal force while allowing fitting and rotation of the blade root in the rotor slot. In these configurations shims both fill the gap and locate the blades in position. Exemplary shims are disclosed in U.S. Pat. No. 6,299,411 B1. A problem with shims is that their production costs are high, partly due to the need for skilled operatives and partly due to the complexity and cost of the shims themselves. In addition, their fitting demands time, impacting blade assembly and disassembly time. JP2004169552A provides an alternative method of blade fixing that involves inserting a spacer between the base of the blade root and channel bottom. A similar spacer used in conjunction with shims is also described in U.S. Pat. No. 3,567,337. As it may not be possible to insert the spacer after the fixing of the blades, the solution increases complexity and in addition does not address the problem of circumferential gaps between roots.
A further alternate locking device, described in GB 2171 150 A, makes use of a bolt and thread to fix the blade into position at a fixed stagger angle.
As an alternative, the solution described in U.S. Pat. No. 7,168,919 B2, provides a blade root with a staggered abutment. During assembly, this abutment enables circumferentially alignment of the root in a way that closes the gap between blades when the roots are in their final operational alignment.
The arrangement is, however, limited to assemblies with shrouded blades in which the blade portions are pre-twisted such that, in the final assembled position, radial alignment of the circumferential abutment and the shroud portions provides a torsional bias that maintains the shroud in pressure and frictional contact with its neighbors. This contact is needed to resist radial movement. Further, the need to overtwist the shrouds of blades fitted with the described blade roots during fitting in order to create the necessary gap to fit the penultimate blade, in view of the require torsional bias, adds installation complexity and as a result impacts assembly time.